1. Field of the Invention
This invention relates to electrical protective devices. More particularly, it relates to devices for protecting communications circuits against excessive voltage surges and excessive currents.
2. Description of the Prior Art
In telephone engineering, it is usual practice to provide protectors at central offices for each incoming line. These protectors, which may be termed modules, combine protection against excessive voltages resulting from lightning, for example, with protection against sneak currents. Sneak currents are not strong enough to do any damage if they flow briefly, but may generate enough heat to char conductor insulation and do other damage if allowed to persist. The sneak currents are produced by voltages of relatively low magnitude as compared to the excessive voltages mentioned hereinabove and usually result from accidental interference between telephone lines and adjacent power lines.
Protection of a telephone line against excessive voltage is usually provided by a so-called spark-gap protector which generally includes a pair of spaced carbon electrodes or a gaseous discharge device. One of the electrodes is usually connected to ground and the other is usually connected to the incoming telephone line. Should a high voltage be impressed on the line, it will bridge the space or gap between the electrodes and cause current to flow to ground, thus bypassing sensitive equipment which is associated with the line.
The second type of protection is commonly provided by a device that is referred to as a heat coil. The heat coil includes a coil of small guage, high resistance wire which is wound on a metal sleeve inside of which a contact pin is held in a predetermined position by a fusible bonding material such as solder, for example. Should excessive currents occur on the line and persist, sufficient heat will be generated by the coil of wire to melt the solder and release the pin. A spring is usually provided which urges the released pin into electrical contact with a source of ground potential to ground the line and protect sensitive line equipment.
A protector assembly of this general type is disclosed in U.S. Pat. No. 2,546,824 which was issued to P. P. Koliss on Mar. 27, 1951. A contact pin of a heat coil subassembly protrudes into a bore that extends through one of two carbon block electrodes of the spark-gap protector and is releasably held in a sleeve by a solder joint. It includes a pair of springs, one of which retains the elements of the assembly in abutting relation. When the pin is released by current buildup in the heat coil that melts the solder joint, the other spring urges the contact pin through the one carbon electrode into engagement with the other electrode which is connected to a source of ground potential.
Inasmuch as a ring conductor and a tip conductor are associated with each telephone station apparatus, each telephone line requires two protector assemblies. A telephone circuit protector module shown in J. B. Geyer et al U.S. Pat. No. 3,573,695 which issued on Apr. 6, 1971 and which is incorporated by reference hereinto includes two protector assemblies enclosed in a single insulative housing to save space, to protect the assemblies from dust, and to facilitate installation. Each protector assembly includes a spark-gap subassembly, having spaced carbon blocks, for excessive voltages and a heat coil subassembly for excessive currents. The spark-gap and heat coil subassemblies are held in abutting aligned relation by a single spring which is part of the normal transmission circuit. The spring also serves to propel a pin of the heat coil subassembly into engagement with a grounding circuit, which includes one of the carbon blocks, during the passage of excessive currents through the heat coil. In Geyer et al, the axis of each heat coil pin is aligned axially with the axis of its associated carbon blocks.
While modules such as those described hereinabove have proved very useful in protecting telephone circuits from excessive voltages and currents, efforts have been made to introduce improvements. For example, to complete a fault current path to ground, the pin in the heat coil subassembly must be brought into contact with a carbon block in the spark-gap protector subassembly. This causes excessive heating of the spark-gap subassembly which becomes part of the fault path. Heat build-up in the carbon blocks of the spark-gap subassembly is commonplace because of their relatively high resistance. A further disadavantage is that the physical arrangement of the heat coil subassembly utilizes excessive space within the protector module. This together with the extension of a contact pin through the voltage protection portions of the protector has precluded the use of gaseous discharge devices in place of carbon blocks. Gaseous discharge devices, which are commonly referred to as gas tubes, are desirable because of their longer lives and because they afford better control of the breakdown voltage.
These last-mentioned problems have been overcome by a protector module shown in U.S. Pat. No. 4,215,381 which issued on July 29, 1980, to R. F. Heisinger and which is incorporated by reference hereinto. The module includes a heat coil subassembly for sensing excessive currents and a voltage surge limiter assembly which is axially aligned with the heat coil subassembly for conducting excessive voltages through a grounding subassembly to a source of ground potential. When excessive currents are encountered in a line circuit, the protector provides a direct metallic contact between the line circuit and ground.
In the Heisinger arrangement, gaseous discharge devices may be used inasmuch as the voltage protection portion of the protector is taken out of the fault circuit. When sufficient heat is transferred to the heat coil subassembly such as by a current fault, a fusible alloy melts to allow a spring to cause a heat coil flange to move and touch a laterally projecting tab of a ground terminal assembly. This creates an electrical path external to the voltage protector subassembly through to the ground terminal assembly. If a prolonged voltage surge occurs, there is an arcing over in the voltage surge limiter assembly, heat energy is transferred to a pin of the heat coil which engages a portion of the voltage surge limiter assembly, the fusible alloy is melted, and the spring moves the heat coil flange plate as before.
Although the Heisinger protector module overcomes the problem of prior art arrangements which precluded the use of gaseous discharge devices for voltage surge protection, it continues to use a spring as part of the normal transmission and fault current circuits. Since the spring moves slidably, insulating sleeves are disposed about the spring to prevent shorting. At times, the presence of the spring in the voice frequency circuit results in noise on the line.
A protector module in which a spring is not in the transmission circuit is disclosed in U.S. Pat. No. 4,168,515. When an excessive circuit increase occurs, a fusible alloy is melted to allow a bobbin on a pin of a heat coil assembly to be moved by the spring. This allows a cup, which is supported indirectly by the bobbin, to be moved by the spring to engage a plate to which the heat coil, line and central office pins are staked. As a result, a fault current path is established from the line pin through the cup to a ground plate.
The aforementioned prior art protector assemblies each include a seemingly excessive number of elements. Since substantial quantitites of these protectors are produced annually, deletion of one or more elements in each protector assembly would result in substantial cost reductions. What is needed is an electrical protective device which is relatively simple with a minimal number of elements, but is one which does not sacrifice the protection afforded to a telephone circuit. Also, the desired device should be one in which carbon blocks or gas tubes could be used interchangeably and one in which there is no spring in the normal transmission circuit.